Can a young man who has recently committed to religion and faced whispers and doubts about the existence of God, then overcome them, get rid of his current whispers and doubts about the truthfulness of Prophet Muhammad, peace and blessings be upon him, especially after learning about the splitting of the moon and the fulfillment of the Roman victory as prophesied by the Qur’an, and is the story of Salman al-Farisi’s conversion to Islam and what is mentioned in Wikipedia about the Jews of Khaybar seeking help from the unlettered Prophet before his mission correct?

What is going through your mind are whispers (waswas) that do not affect the soundness of your faith. The cure for them is to seek refuge with Allah and to stop dwelling on them. The proofs of the Prophet's truthfulness, peace be upon him, are as clear as the sun. Sufficient proof for him is the Noble Quran, which has defied creation to produce anything like it. As for his miracles, his foretelling of the unseen, and his mention in previous scriptures, they are numerous, and you can refer to the book "The Messengers and the Messages" by Dr. Omar Suleiman Al-Ashqar. As for the story mentioned, it was narrated by Al-Hakim and Al-Bayhaqi with a weak chain of transmission.